Analyte Guru

Capillary Ion Chromatography Improves Forensic Analysis

Recently, we presented an online webinar on how capillary ion chromatography (IC) is being to improved forensic analysis. The speaker was Dr. Leon Barron, Lecturer in Forensic Science, Forensic Science and Drug Monitoring Analytical & Environmental Science Division School of Biomedical Sciences, King’s College London. Dr. Barron discussed analytical methods he has developed at King’s College London, in collaboration with the London Metropolitan Police, to determine inorganic tracer species in forensic samples such as shell casings, latent fingerprints, and human sweat. He also talked about his use of traditional-scale and capillary scale IC, and explained the advantages associated with the use of capillary IC technology.

Webinar Q&A

Note: This Q&A has been edited minimally for readability.

Laura Bush (LB): What weights of anions detected were present in fingerprints?
Dr. Barron (DB): The weights detected in the fingerprints vary between individuals: we saw orders of 13 ng to about 2.7 µg. There might be an even wider expanse as this was just the small sample set we had.

LB: What concentration sensitivities could you achieve with the two systems?
DB: A lot of people considering the capillary system will think that it will give you enhanced concentration sensitivity. The micro-bore method that we developed, the range of concentration sensitivities we got were between 3 and 88 µg per liter for all species and the capillary system was relatively similar, or be it just a little bit lower on the front end with .7 to 64 µg per liter.

So, the concentration sensitivities are quite similar, and the advantage really is the mass sensitivity because of the GSR and the explosive residues left on a surface. We also consumed very small amounts of eluent over the time period of the study.

LB: Did you rinse the swabs before analyzing their ion content?
DB: We analyzed dry swabs and swabs rinsed with deionized water. There was not a really significant change in ion concentrations: it got a little bit cleaner. The cotton swabs are commonly used to collect samples for IC as well as in forensic case work, so I think that this study shows that maybe other collection methods should be considered, especially using IC to prevent contamination.

LB: Why did you analyze drinking water?
DB: My background is in environmental science and I did my PhD on drinking water. So we did quite a lot of work at that time period on trace species in drinking water and one of the things I noticed from reading papers is that a lot of the studies, or some of the older studies used nitrite or nitrate as target species for explosives, but these regularly occur in environmental samples and were present in our main supply as well.

Therefore, we ran the drinking water sample just to give us an environmental sample to compare to. Obviously that represents a fairly clean sample for an environmental sample, as compared to other samples, such as rain water or river water for instance that would be a lot more heavily contaminated with ions.

LB: In your study, how long was it before the fingerprints were analyzed?
DB: The fingerprints were taken and extracted immediately. One of the things with the fingerprints is that as soon as the fingerprint is deposited it starts to lose water mass, so we wanted to prevent evaporation.

We looked at a 10 min extraction of the fingerprint on an analytical balance and 500 µL of water deposited with a capette. We did see some loss of total water volume of about 8% and that was corrected for, and then included in the calculations of the concentrations we observed.

LB: What type of sample preparation is required for the use of cotton swabs with IC?
DB: Because we are taking a sample from a surface, we just wipe the surface with the swab. Sometimes it is pre-wet, sometimes we use a series of dry followed by wet, or wet followed by dry and then it is extracted in ultrapure water to minimize any contamination from the extraction solvent for about 10-15 min to get some appreciable sensitivity. We are more interested in the ion ratio than absolute measurements.

LB: Do you think you could apply your fingerprint technique to dust-lifted fingerprints?
DB: First, we would have to eliminate the dusting material used. This is one of the hot topics that we are looking to do. I would say that certainly we want to see since we are just putting a droplet of water onto a fingerprint directly, we would like to see the persistence of that print before it is extracted. Obviously, it is hard to see the latent print on the surface, but if it is non-destructive of the print than you could possibly develop it afterwards.

Please add any questions you might have after viewing the webcast below in the Comments box. We look forward to hearing from you.